.oberve(.childAdded) producing error while .observeSingleEvent(of: .value) is not - swift

Question

Right now I have .observeSingleEvent(of: .value) that loads all my annotations onto the map:
func loadAnnotations() {
if let uid = Auth.auth().currentUser?.uid {
uidRef.child(uid).child("annotations").observeSingleEvent(of: .value, with: { (snapshot) in
for item in snapshot.children {
// annotationListItem is a struct I created
let annotationItem = AnnotationListItem(snapshot: item as! DataSnapshot)
let doubleLatitude = Double(annotationItem.mapViewLatitude!)
let doubleLongitude = Double(annotationItem.mapViewLongitude!)
let coordinate = CLLocationCoordinate2DMake(doubleLatitude!, doubleLongitude!)
let annotation = MKPointAnnotation()
annotation.coordinate = coordinate
annotation.title = annotationItem.annotationTitle
annotation.subtitle = annotationItem.annotationSubtitle
self.mapView.addAnnotation(annotation)
}
}, withCancel: nil)
}
}
Now I want the map to update every time the user adds a new annotation so I put in the exact same code but with .observe(.childAdded)
func annotationChildAdded() {
if let uid = Auth.auth().currentUser?.uid {
uidRef.child(uid).child("annotations").observe(.childAdded, with: { (snapshot) in
for item in snapshot.children {
let annotationItem = AnnotationListItem(snapshot: item as! DataSnapshot)
let doubleLatitude = Double(annotationItem.mapViewLatitude!)
let doubleLongitude = Double(annotationItem.mapViewLongitude!)
let coordinate = CLLocationCoordinate2DMake(doubleLatitude!, doubleLongitude!)
let annotation = MKPointAnnotation()
annotation.coordinate = coordinate
annotation.title = annotationItem.annotationTitle
annotation.subtitle = annotationItem.annotationSubtitle
self.mapView.addAnnotation(annotation)
}
}, withCancel: nil)
}
}
I get the error of:
Could not cast value of type '__NSCFString' (0x1060b84f0) to 'NSDictionary' (0x1060b92d8).
Printing description of snapshotValue:
([String : AnyObject]) snapshotValue = variable not available>
How can I fix this problem?
UPDATE
.observe(.value) works. But I am still wondering why .childAdded doesn't

Answer 1

The problem here is that .observeSingleEvent(of: .value) and .observe(.childAdded) don't return the same thing.
When you call .observe(.value) it returns the entire node with all the content inside at the time of the event.
But, when you use .observe(.childAdded) it would return only the content that is added to the specified path (that is, the child that is added).
You can see that by doing print(snapshot) in both methods and you will easily see the difference.
So, to access your data using .childAdded, you don't need to iterate through all the children like you would do with .observeSingleEvent(of: .value). Instead you would do :
guard let uid = Auth.auth().currentUser?.uid else {
return
}
uidRef.child(uid).child("annotations").observe(.childAdded, with: { (snapshot) in
let annotationItem = AnnotationListItem(snapshot: item as! DataSnapshot)
let doubleLatitude = Double(annotationItem.mapViewLatitude!)
let doubleLongitude = Double(annotationItem.mapViewLongitude!)
let coordinate = CLLocationCoordinate2DMake(doubleLatitude!, doubleLongitude!)
let annotation = MKPointAnnotation()
annotation.coordinate = coordinate
annotation.title = annotationItem.annotationTitle
annotation.subtitle = annotationItem.annotationSubtitle
self.mapView.addAnnotation(annotation)
}
})
Also, I would not recommend you to force cast your item like that item as! DataSnapshot because if you miss something in your database the app would just crash.
Instead, I would do it like so, using guard statements :
guard let uid = Auth.auth().currentUser?.uid else {
return
}
uidRef.child(uid).child("annotations").observe(.childAdded, with: { [weak self] (snapshot) in
let annotationKey = snapshot.key
guard let longitude = snapshot.childSnapshot(forPath: "longitude").value as? NSNumber else { return }
guard let latitude = snapshot.childSnapshot(forPath: "latitude").value as? NSNumber else { return }
guard let title = snapshot.childSnapshot(forPath: "title").value as? String else { return }
// Here it really depends on how your database look like.
// ...
// Create your annotation item using the values from the snapshot :
let annotation = AnnotationListItem(id: annotationKey, title: title, longitude: longitue, latitude: latitude)
// Finally add the data to an array :
self?.annotationsArray.append(annotation)
// Or directly to your mapView :
self?.mapView.addAnnotation(annotation)
})
Just let me know if it helps ;D
Update
For example, let's say you have initially 3 child nodes in your database. It would do something like that :
initial : observer get called :
you get child 1
you get child 2
you get child 3
now, if another child is added :
new child : observer get called :
you get child 4
etc..